This invention relates to a method of passivating stainless steel alloys. More particularly it relates to passivating stainless steel surfaces for polymer handling equipment such as spinning pack parts.
While stainless steel alloys by their very nature are considered to be corrosion resistant, it has been found that under certain conditions stainless steels possess a certain reactivity detrimental to the material coming in contact with it. For example, when used for spinnerets to extrude molten polymer through orifices in the spinnerets corrosion products will build up around the edge of the orifices causing polymer drips to form on top of spun filaments adversely affecting the product.
Prior art treatments of spinneret materials have included the use of heat treatment to form a thin oxide coating, increased hardness, and consequently, better corrosion resistance. Tegart points out in his book (The Electrolytic and Chemical Polishing of Metals, pp. 108-109, 1959) that Tegart solution, although meant for chemical polishing, when used on alloy steels will result in coating of a passivating film. Mahla and Nielsen (Trans. Electrochem. Soc. Vol. 89, pp. 167-194, 1946) treated polished (clean) surfaces in nitric acid, potassium chromate solution and found that such treatment merely delays the onset of corrosive activity.